The present invention relates to a surface acoustic wave device for use in a communication system, for example as a filter, and more particularly, to an electrode arrangement of the surface acoustic wave device which makes it possible to reduce or eliminate unwanted reflected waves, such as triple transit echo waves, without increasing the insertion loss.
Generally, a surface acoustic wave (SAW) device comprises a transmitting, or launching, transducer and a receiving transducer, which are formed from comb-like multi-electrode elements with their teeth interdigitated and disposed on a piezoelectric substrate. When an alternating electrical potential is applied to the electrodes of the transmitting transducer, an alternating electric field is generated that causes localized vibration in the substrate material. The vibrations give rise to acoustic waves, which propagate along the surface of the substrate in a defined path orthogonal to the electrodes, and may be detected at any point along the path by the receiving transducer.
At the receiving transducer, part of the acoustic wave energy is converted to electrical energy and delivered to the load, part of the acoustic wave energy is transmitted past the receiving transducer, and part of the acoustic wave energy is reflected back along the original path towards the transmitting transducer. This reflected surface wave, which is identical in frequency to the original surface wave but smaller in magnitude, is again similarly reflected at the transmitting transducer back along the same path towards the receiving transducer. The surface acoustic wave which has been so reflected twice and which has traveled three times between the transducers is generally called triple transit echo (TTE) wave. Since the TTE wave tends to interfere and distort the main, desired signal, adversely affecting the performance of the SAW device, it should preferably be eliminated. The interference and distortion by the TTE wave may become more considerable when each transducer is coupled with a tuning coil which is normally provided to minimize the insertion loss of the SAW device.
To solve this problem, there have been proposed various methods. One method is shown in FIG. 1, and includes the use of first, second and third transducers 1, 2 and 3 on a rectangular piezoelectric substrate 6. The first transducer 1 has a width, as measured in a direction transverse to the direction of wave propagation, equal to or larger than the combined widths of the second and third transducers 2 and 3, and is located at one end portion of the substrate 6. The transducers 2 and 3, which have identical size and configuration to each other, are located at the other end portion of the substrate 6 in side-by-side relation to each other, and are mutually offset in a direction orthogonal to the direction of surface acoustic wave propagation. Accordingly, the propagation of acoustic surface waves between the longitudinal the transducers 1 and 2 and the propagation of acoustic waves between the transducers 1 and 3 are carried out through different paths 4 and 5, respectively. The distance L.sub.12 between centers of the first and second transducers 1 and 2 differs from the distance L.sub.13 between the longitudinal centers of the first and third transducers 1 and 3 by an odd multiple of one-fourth of the wavelength .lambda.o of the acoustic surface waves at the center frequency of the device. When the transducer 1 is actuated to transmit surface acoustic waves along the paths 4 and 5, part of the surface acoustic wave arriving at the transducer 2 is converted to an electric signal, part is transmitted past through the transducer 2 and part is reflected along the original path towards the transducer 1. Similarly, part of the surface acoustic waves arriving at the transducer 3 is reflected back along the original path. Since there is a difference between distances L.sub.12 and L.sub.13, the acoustic surface wave reflected from the transducer 2 has a phase opposite to that reflected from the transducer 3. Therefore, the two reflected waves with opposite phase will cancel each other during their travel back to the transducer 1. This cancellation of the reflected waves can be effectively carried out even when the tuning coil is coupled to each transducer.
Although the arrangement of FIG. 1 effectively eliminates the undesirable reflected surface wave to prevent any TTE waves from being transmitted to the receiving transducer 2, it is necessary to provide two parallel paths 4 and 5. Thus, the conventional SAW device described above requires a relatively large substrate 6, resulting in high manufacturing cost.
Another method is disclosed in Japanese Utility Model application laid open publication No. 4647/1979 of ONISHI et al. in which a multistrip coupler is employed between transducers, e.g., between transducer 1 and transducers 2 and 3 of the device shown in FIG. 1. According to this arrangement, it is possible to reduce the size of the transmitting transducer 1 to a size similar to those of the transducers 2 and 3. However, this arrangement also has a considerably large size of substrate since the transducers are arranged at positions mutually offset in a direction orthogonal to the direction of acoustic surface wave propagation.
A further method is disclosed in U.S. Pat. No. 3,596,211 to Dias et al. wherein three transducers aligned in a row are used. The center transducer and one side transducer are respectively provided for transmitting and receiving the surface acoustic waves, or vice versa, while the remaining transducer on the other side is provided for producing a reflected surface wave. According to this prior art, the surface waves reflected at opposite side transducers are directed towards the center transducer in which the received reflected waves are converted to electrical signal. Since the distance between the center transducer and one side transducer and the distance between the center transducer and the other side transducers are prearranged relative to the wavelength, the electrical signal created by the reflected signal from one side transducer has a polarity opposite to the electrical signal created by the reflected signal from the other side transducer, resulting in cancellation of the two reflected waves. Therefore, according to this prior art, the cancellation is carried out in the center transducer.